450 SCIENCE PROGRESS 



of the fish becomes zigzag, and the animal wobbles." ^ Yes, 

 and most of us would "wobble" if similarly important pieces 

 of our anatomy were removed. 



The anal or ventral fin and the hinder of the two dorsal 

 fins are accessories of the tail fin, they are always situated in 

 the tail region and give this a greater purchase on the water. 

 The front dorsal fin has a quite different function, the importance 

 of which is indicated by the fact that it is the largest median 

 and often the only dorsal fin and that its front border is 

 frequently made very strong by means of stout spiny rays. 

 A cursory glance at a fish that is swimming rapidly is sufficient 

 to show that this fin is not in use at such times but lies close 

 along the back like the ears of a hare running at full speed. 

 The moment, however, that the fish makes a sudden turn to 

 the side it is erected to the fullest extent. Evidently its function 

 is associated with the turning movement. 



It will usually be found that the strongest and highest part 

 of the front dorsal fin is situated at the point where the body 

 is deepest, and that the vertical line which passes through the 

 fin and the body here, is the one referred to above as a fulcrum. 

 The great vertical depth makes this part of the body less liable 

 to lateral movement than any other part ; thus a suitable axis 

 is provided around which the tail and snout may rotate when 

 the fish attempts to turn rapidly even in a confined space. 

 The power to execute such rapid turning in a small space 

 is essential to the welfare of a fish that lives in streams and 

 shore waters where plants, stones, rocks and other obstructions 

 abound. 



The current notions upon the function of the paired fins 

 may be gathered from the following quotation: "The dorsal 

 position of the centre of gravity renders the equilibrium of 

 the body unstable, and were it not for the balancing action of 

 the paired fins the fish would float belly upwards, as is always 

 the case after death." Again : " The paired fins may act as 

 lateral keels in steadying the course of the fish especially 

 when the fins are extended and their plane horizontal. They 

 certainly seem to act as balancers in keeping the fish on an 

 even keel and in counteracting the tendency of the fish to turn 

 belly upwards, a result which is attained by slight upward and 

 downward movement of the fins, particularly of the pectoral 

 ' Cambridge Natural History^ vol. Fishes, p. 350. 



